A flexible magnetic disk sheet is commonly provided with a jacket having central circular openings aligned with a central circular opening in the sheet. In use, the central circular opening of the sheet is engaged with a positioning member of the feeding and reading device adapted to the sheet, by which magnetic recording on the sheet or magnetic reproduction from the sheet is carried out by revolving the sheet in careful alignment. In such a magnetic disk, since the sheet is enclosed in a jacket with a certain allowance permitting free movement, it shifts position in the jacket, during storage, handling or carrying. When the disk is fitted to the drive device, the circular opening in the center of the sheet may not be in alignment with a positioning member (collet) of the drive device and the sheet may thus be held by the positioning member in an off-center or eccentric position.
FIGS. 1A and 1B show such a flexible magnetic sheet, wherein circular flexible magnetic sheet 2, in which the rim of the central circular opening 2a is exposed, is contained revolvably in rectangular jacket 1 having circular opening 1a in the center.
When this flexible magnetic disk is fitted to the feeding and recording device, sheet 2 is supported by revolving part 3 of the positioning member as shown in FIG. 2, and is held in position by a collet 4 which extends through central circular opening 2a into a circular hollow part 3a of revolving part 3. However, the rim of the central circular opening 2a of the sheet is not always properly aligned with the position of the circular hollow part 3a of the revolving part 3, but frequently deviates. In this case if collet 4 is pressed down on the misaligned sheet, a part of the rim of the circular opening 2a of the sheet 2 is caught between the collet 4 and the hollow part 3a of the revolving part 3 as shown in FIG. 3, and the disk revolves in an eccentric manner. Since proper feeding and reading can only be carried out, if the magnetic disk sheet 2 revolves in a circular fashion, having its center coincident with the revolving axis, it must be kept in such alignment. Further, if the sheet is held by the drive off-center, the flatness of sheet 2 is reduced and a problem of shaking occurs during revolution. In order to correctly align the sheet 2 by pressing down collet 4, as shown in FIG. 4, and to avoid misalignment and crimping of the rim of the circular opening as shown in FIG. 3, the rim of the central circular opening of the sheet must have sufficient hardness, and the coefficient of friction between sheet 1 and collet 4 and revolving part 3 must be small.
For this purpose, U.S. Pat. No. 4,387,114 discloses a mwethod of reducing the coefficient of friction, by providing a protective layer containing a polyolefin material capable of polymerization by ultraviolet rays and an aliphatic acid ester on the rim of the central circular opening which is in contacts with the positioning manner. However, a lubricant having a low molecular weight is used in this method, which has the disadvantage that precision of disc alignment is reduced when the coefficient of friction thereof increases at a comparatively high temperature of 40.degree. C. to 50.degree. C. under a high humidity of 70 to 80%. In addition, the effect of lubrication diminishes when fitting is repeatedly carried out at room temperature, which is believed to be caused by removal of the lubricant from the surface upon repeated use.